Process of producing high-strength ore pellets



United States Patent 3,433,625 PROCESS OF PRODUCING HIGH-STRENGTH ORE PELLETS Hermann Schenck, Intzestrasse 1, and Werner Wenzel, Prinz-Heinrich-Strasse 29, both of Aachen, Germany No Drawing. Continuation of application Ser. No. 475,015, July 26, 1965. This application Sept. 14, 1967, Ser. No. 667,867 Claims priority, application Germany, Aug. 1, 1964, Sch 35,572; Mar. 22, 1965, Sch 36,747 US. Cl. 75-5 12 Claims Int. Cl. C21b I I 0 ABSTRACT OF THE DISCLOSURE There is disclosed a method for the production of high strength iron ore pellets by oxidizing-hard firing at a temperature in the sintering temperature range of ore in which connection prefired ore is added to the unfired ore, the prefired ore being present in amounts of to 50% based on the dry weight of the resulting pellet and the sintering of this mixture carried out so as to produce individual hard strength ore pellets free from any sign of caking. The invention also includes the novel pellet thus produced.

This application is a continuation of application Ser. No. 475,015, filed July 26, 1965, now abandoned.

The present invention relates to high-strength ore pellets and methods for making the same. More particularly, this invention relates to improved high-strength iron-ore pellets and an improved process for manufacturing such pellets.

In known processes, high-strength ore pellets suitable, e.g., for being charged to a blast furnace, are produced by finely grinding the ore and transforming the same in moist condition into pellets on a pelleting machine, whereafter these pellets are heated by known means to a high temperature for a thermal consolidation of the pellets. In many cases, artificial consolidating agents, such as bentonite, are added to the pellets in small percentages.

The strength of the resulting pellets depends on the fineness of the grain of the material forming these pellets, the amount of admixtures, such as bentonite, and their properties, the firing temperature and the firing time. All these measures add to the cost of the produced pellets. For this reason it is an important object to produce highstrength ore pellets in which the degree of grinding, the firing temperature and the firing time are as low or short as possible or the expenditure involved in at least one of these measures can be reduced. From the metallurgical aspect, an addition of pelleting admixtures, such as bentonite, is undesirable because in iron ore pellets, e.g., it reduces the iron content of the pellets.

Another process is known in which fired pellets are tumbled in a barrel before they are used in a blast furnace. This tumbling serves to smoothen the pellet surfaces. The rolling operation is intended to break off the protruding portions of the material. This rolling produces a small percentage of waste, which is recirculated to the ground material to be pelletized. This addition of recirculated material has virtually no influence on the strength of the pellets.

It is therefore an object of the present invention to overcome these and other difficultties encountered in the prior art. It is a further object of this invention to provide highstrength ore pellets. Another object of this invention is to provide a method for making such pellets at a reduced cost. Additionally, it is another object of this invention to produce such pellets without the addition of bentonite and equivalent materials, and in which either the degree of grinding, or the firing temperature and the firing time, or all are reduced.

The present process teaches how high-strength pellets can be produced with a reduction of the expenditure involved in at least one of the above-mentioned measures adopted in producing high-strength pellets. It has been found that high-strength pellets can be obtained if prefired material, e.g., so-called recirculated material, is added to the ore to be pelletized in a percentage which is much higher than the amount added in the known process, e.g., in an amount of about 10-50%, preferably 2040%. This prefired material may consist of prefired and subsequently disintegrated pellets, or of fired and prereduced pellets, which are disintegrated to the pelletizing particle size, or of sintered material, which may be obtained as a finegrained waste in the known belt sintering process and which is also disintegrated to the pelletizing particle size. Alternatively, the admixture may consists of a fine-grained ore, which has been heated to a temperature in or slightly below the sintering temperature range while maintaining its particle size and is then admixed in a cold condition to the material to be pelletized.

In this specification and the appended claims, the amount of recirculated or prefired material which is added to the crude material to be pelletized, unless otherwise expressed, is based on the dry weight of the resulting mixture.

The process according to the invention will be explained in the following non-limiting examples:

EXAMPLE 1 An ore containing 64% iron, specifically 90% Fe 0 3.6% SiO 1.7% A1 0 balance CaO-i-MgO-l-MnO, was disintegrated to the following screen analysis:

Mm. Percent Above 0.30 1 015-03 15 0.09-0.15 18 0.06-0.09 29 Below 0.06 37 The pellets made from this ore were fired at 1170 C. for 60 minutes. The resulting crushing strength was 65 kg.

EXAMPLE 2 Pellets were made as in Example 2 but from a mixture containing 40% recirculated sintered material. The crushing strength was 260 kg.

According to this example the process according to the invention provides a desirable combination of sintering and pelletizing means. The fines which are recirculated from the sintering means are suitably fed to the pelletizing means rather than being resintered.

If the recirculated material required for pelletizing is to be obtained from the fired pellets, these pellets are suitably subjected to an appropriate stress so that only the hard pellets, which have the highest strength, are left intact for being charged to the blast furnace Whereas the pellets which do not resist the stress are recirculated.

This stress may be applied, e.g., by dropping the fired pellets onto an iron plate, or in a tumbling barrel similar to a Micum barrel. Two such stressing steps may be carried out in succession so that, e.g., the dropping of the pellets causes cracks to form in some of the pellets and these cracks cause these pellets to break apart during the subsequent tumbling.

Whereas in the known process of smoothing the pellet surfaces the recirculated material has virtually no influence on the strength of the resulting pellets, the addition of about -50% recirculated material according to the invention results in a substantial increase in the strength of the pellets. The production of this recirculated material requires a much higher stress than is applied in the known smoothing process.

In the production of high-strength pellets from some ores, an addition of about 4080% prefired ore, based on the amount of crude ore, in the mixture to be pelletized is required. This large amount of prefired material, such as distintegrated recirculated material, is not desirable because the effective throughput is reduced and the disintegration of the required amount of recirculated material involves considerable costs.

According to a further embodiment of the invention, this disadvantage is overcome in that only the outer shells of the pellets are provided with an admixture of 4080% prefired material whereas the addition of prefired material is only l5-50%, based on the total amount of crude ore. A smaller proportion of prefired ore or no prefired ore at all is contained in the interior of the pellets.

Such pellets having a hard shell may be produced as follows according to the invention:

The content of prefired ore, e.g., disintegrated pellet material, is about e.g., which means that 15 parts of prefired ore have been added to 85 parts crude ore. These 15 parts of prefired ore are mixed with 15 parts of crude ore. As a result, there are 70 parts of unmixed crude ore and 30 parts of a mixture which contains 50% of prefired ore and results in particularly hard pellets when fired. Using a known pelletizing machine, such as a pelletizing plate, pellets are first made from the 70 parts of crude ore. A layer of the 30 parts of the mixture of crude ore and prefired ore is then pelletized onto these pellets. Firing results then in pellets having a hard shell and a softer core. When the pellets are subsequently stressed during the reducing process, e.g., in the blast furnace, the thick shell of hard-fired material protects the softer core.

In pellets which are subjected to high pressure, it is not desirable for the hard shell produced according to v the invention to surround the softer core without a transition zone because the shells may then crack or even burst off.

For this reason, a further development of the invention resides in the production of pellets in which the hardness changes gradually from the outer shell, having the highest hardness to the soft inner core. This gradual transition will be obtained if the proportion of the admixed prefired material in the pellet increases gradually from the inside to the outside.

Such pellets may be made, e.g., on a normal pelletizing plate if the same is successively charged with mixtures of crude ore and increasing proportions of prefired material. Even if the proportion of prefired material increases in steps, there will be no sudden increase in the proportion thereof in the subsequently applied .layers of the pellets if care is taken that at the time when the new mixture is added the preceding mixture which is on the plate has not yet been completely pelletized. For instance, when pellets having an average final diameter of 10 mm. are made on a pelletizing plate, the invention may be practiced by initially making pellets having a diameter of about 7 mm. from crude ore on the pelletizing plate. At a time when about 1020% of the amount of crude ore on the pelletizing plate have not yet been pelletized, a mixture containing 25% prefired material is charged onto the plate in such an amount that a layer about 0.5 mm. thick is formed on the pellets, which layer has an average content of about 15% prefired material when about l020% of the material on the plate are nonpelletized. Then a mixture containing about 40% prefired material is charged onto the plate in such an amount that a pelletized layer is formed which is 0.5 mm. thick and has an average content of 20% prefired material. This is followed by a layer which is 0.5 mm. thick and contains about 30% prefired material. The outermost and hardest layer is 0.25 mm. thick and contains about prefired material. Thus, the pellets are provided with a layer which has a total thickness of about 1.5 mm. and a gradual increase in hardness from the inside to the outside, whereas the admixture of prefired material, based on the final pellets, is between 15 and 20%.

Shaped bodies made from fine-grained ores, particularly ore pellets, which have outer shells that have been hardened by special measures, and a softer core, are known per se. It is not known, however, to effect the hardening of the outer shells in a surprisingly sim le manner by admixing prefired material and subsequently firing at a temperature in the sintering temperature range of the mixture, as is taught according to the invention.

The production of pellets which comprise layers containing different amounts of recirculated material will be described more specifically in the following example.

EXAMPLE 4 The ore which was charged had the following analysis: Percent Fe 63.8 FeO 1.24

F6203 Fe MnO 0.24 CaO 0.15 MgO 0.50 A1 0 1.69 Si0 3.56 P 0.04 S 0.085

By screen analysis, the ground ore contained 40% by weight of particles smaller than 0.04 mm.

The pellets were fired at 1200 C. The material which was recirculated from said fired pellets had the same analysis as the crude ore and was ground so that it contained by screen analysis 40% by weight of particles smaller than 0.04 mm. Its particle size composition matched that of the starting ore.

Ore which contained no recirculated material was initially charged onto the pelletizing plate. Pelletizing was interrupted when the pellets were about 7 mm. in diameter and about 10-20% of the charged ore on the plate were non-pelletized, which means that they were only slightly agglomerated, having a particle size below 1 mm.

Additional material to be pelletized is added to this mixture of pellets and slightly pelletized material. This additional material consisted of a mixture of about 75% ore and 25% prefired material (recirculated material). This additional material is supplied in such an amount that a layer having an average content of 15 recirculated material is formed on the pellets in a thickness of about 0.5 mm. Pelletizing is again interrupted when this layer has the desired thickness. The amounts are so selected that about 10-20% surplus material which is not pelletized are left on the plate. A mixture of ore and 40% recirculated material is now added onto the pelletizing plate and is pelletized to form a layer which has a thickness of 0.5 mm. and contains 20% recirculated material. In a similar manner, a layer which has a thickness of 0.5 mm. and contains 30% recirculated material is formed on the pellets as well as a final layer, which has a thickness of about 0.25 mm. and contains 50% recirculated material.

The pellets are thus provided with a layer which has a total thickness of about 1.5 mm. and in which the content of recirculated material increases gradually from the inside to the outside. The resulting pellets contain 18% recirculated material. The pellets produced by this process are fired for half an hour at 1200 C. in an oxidizing atmosphere. After cooling, they have a point strength of 130 kg. Pellets of the same size made from the same ore without recirculated material and fired in the same manner have a point strength of only 80 kg.

Although the invention has been described with reference to certain preferred embodiments, it is not intended that such description be construed as limiting the novel method and article of manufacture of the invention, and certain modifications are intended to be included within the broad scope of the following claims.

What is claimed is:

1. Method of producing discrete pellets of iron ore which comprises admixing about to 50 weight percent prefired ore with unfired ore, wherein said prefired ore has been produced by firing pellets of iron ore and fragmenting such pellets; pelletizing such mixture and hard firing such pellets at a temperature up to about the sintering temperature of said ore, which temperature is insufiicient to sinter said pellets together.

2. The improved method claimed in claim 1, wherein said prefired ore is present in a proportion of about 20- 40 weight percent.

3. The improved process claimed in claim 1, wherein said prefired ore is sintered ore which has been produced by a downdraft sintering process.

4. The improved process claimed in claim 1, wherein said prefired ore comprises fine-grained ore preheated to a temperature from slightly below its sinterin temperature to its sintering temperature while maintaining the particle size of said fine-grained ore substantially constant during said preheating.

5. The improved process claimed in claim 1, including prefiring said prefired ore under reducing conditions.

6. Improved process claimed in claim 1, wherein said fragmented prefired ore pellets are admixed with unfired ore prior to pelletizing such mixture, in substantially the size range caused by such fragmentation.

7. Iron ore pellets comprising prefired ore and unfired ore wherein said prefired ore constitutes about 10 to Weight percent of said pellet, and wherein said pellet has a concentration gradient such that the proportion of prefired ore decreases from the outside to the inside of the pellet.

8. An iron ore pellet as claimed in claim 7, having a core which is substantially free of prefired ore.

9. An iron ore pellet comprising :an admixture of prefired ore and unfired ore containing about 10 to 50% of said prefired ore, wherein said prefired ore is fragmented hard fired iron ore pellets.

10. The product claimed in claim 9, wherein said mixture contains about 20-40 weight percent prefired ore.

11. The article claimed in claim 9, wherein said prefired ore is sintered ore produced by a downdraft sintering process.

12. The article claimed in claim 1, wherein said prefired ore has been prefired under reducing conditions.

References Cited UNITED STATES PATENTS 2,543,898 3/1951 De Vaney -3 2,750,273 6/1956 Lellep 75-3 2,799,572 7/1957 Holt et' al. 75-5 3,003,864 10/1961 Kraner et a1 75-3 3,083,090 3/1963 Davies 75-5 3,134,667 5/1964 Holowaty et al 755 L. DEWAYNE RUTLEDGE, Primary Examiner. ERNEST L. WEISE, Assistant Examiner. 

